Datsun 720 Continues To Run After Being Turned Off

[fragginfags]

As the title suggests, once i turn the car off the engine continues to run HORRIBLE, it shakes the whole car sometimes for 3 seconds and other times for about 30, it just shakes violently before stopping, im new to the whole engine side of things and im wondering if anyone has had this problem before and how did they fix it? any help would be appreciated

[datzenmike]

It's called run on or dieseling. It's what happens when the engine is hot and there is enough fuel and air to fire the cylinder simply from the heat, just like a diesel.

 

This can be from

Too fast an idle

Spark plug to high a heat range or wrong kind.

Too rich a mixture causing carbon build up. The carbon becomes glowing hot.

 

Too rich can be wrong jet size, dirty air filter, fuel pump pressure too high for the carb, floats incorrectly set.

 

Easiest thing to do, and get used to doing when shutting off the ignition, is hold brake pedal down. Lift clutch to load the engine down slightly. Turn ignition off. The load on the engine is too much for run on or dieseling to keep turning against and stalls. When completely stopped, release clutch completely. Done. A half dozen times and it becomes a natural thing to do and if you're good, not even noticeable to passengers. An automatic is easier... turn ignition off while in drive. Stall engine and then place in park.

[banzai510(hainz)]

if a weber carb has been installed with no idle cut off this is common.

try running super gas

[Tinman]

And if it's the Hitachi carb, has someone removed plunger from the idle stop solenoid? 

 

I've see this on a few used carbs I picked up.  Rather than fix a problem, people just pull out the plunger and screw the solenoid back in.

[datzenmike]

What year 720?

Does it have the electric feedback carb? (does it have an O2 sensor in the exhaust manifold?)